Fewer than one quarter of Metrobuses were equipped with the shields as of Tuesday, a Metro representative said. Of the 1,542 buses Metro operates, 326 buses have the shields. Another 531 buses will have the safety devices by the end of 2016.

The shields are typically used in areas where drivers have previously had trouble, the Metro representative said.

"This union, representing more than 9,000 of the operators, station managers, clerks and mechanics of Metro, is calling upon Metro to take action and immediately increase police presence," Amalgamated Transit Union Local 689 said in a statement. "The lives of transit employees and our riders are depending on it."

Metro deploys officers based on where they see spikes in crime. Attacks on drivers have declined by about 30 percent in the past year, the Metro representative said.

A Metro official not authorized to speak on camera said Metro officials and union officials are working on new safety training and protocols.